1. Field of the Invention
The present invention relates to an oscillator circuit used for local oscillation of, for example, a television tuner.
2. Description of the Related Art
FIG. 4 shows an oscillator circuit 1 according to the related art that is used, for example, for a low or high band of a VHF television tuner. The oscillator circuit 1 includes first and second oscillating transistors 2 and 3 provided in an integrated circuit 20 and a resonance circuit 4 provided outside the integrated circuit 20. Emitters of the first and second oscillating transistors 2 and 3 are connected to each other and are connected to a constant current source 5. Further, a collector of the first oscillating transistor 2 is connected to a power supply terminal B of the integrated circuit 20, and a collector of the second oscillating transistor 3 is connected to the power supply terminal B through a feeding resistor 6. Furthermore, the collector of the first oscillating transistor 2 and the base of the second oscillating transistor 3 are respectively connected in a high-frequency wise to a common ground point in the integrated circuit 20 by a first DC cutting capacitor 7 and a second DC cutting capacitor 8 which are provided in the integrated circuit 20.
Therefore, the first and second oscillating transistors 2 and 3 are differentially connected to each other, and the base of the first oscillating transistor 2 and the collector of the second oscillating transistor 3 are similar to each other in phase.
The integrated circuit 20 is provided with a terminal 20a. Further, first and second coupling capacitors 9 and 10 are provided in the integrated circuit 20. The base of the first oscillating transistor 2 is connected to the terminal 20a through the first coupling capacitor 9, and the collector of the second oscillating transistor 3 is connected to the terminal 20a through the second coupling capacitor 10. The resonance circuit 4 has an inductance element 4a, a varactor diode 4b, etc. One end of the resonance circuit is connected to the terminal 20a, and the other end of the resonance circuit is grounded outside of the integrated circuit 20.
Therefore, the oscillator circuit 1 forms an unbalanced oscillator circuit. An oscillation frequency is set on the basis of a tuning voltage Tu to be applied to the varactor diode 4b (for example, see JP-A-2004-172934 (FIG. 1)).
The varactor diode 4b of the resonance circuit 4 has a capacitance value with a positive temperature coefficient. Therefore, the resonance frequency of the resonance circuit has a negative temperature coefficient so as to decrease as temperature rises. In particular, when the voltage between both ends of the varactor diode is low, the capacitance value increases, and thus the frequency varies over a wide range.